GAZA/JERUSALEM, July 12 (Reuters) - Israel rushed an eighth missile interceptor battery into service on Saturday to counter stronger-than-expected rocket fire from Gaza as the military pounded positions in the Palestinian enclave for a fifth day, killing 19 people, medics said.

The Jewish state kept options open for a possible ground offensive into densely populated Gaza despite international pressure to negotiate a ceasefire in the conflict, which has killed 125 people in the Islamist-ruled enclave since Tuesday.

The U.N. Security Council, after days of discussion, issued a statement calling for a ceasefire and expressed serious concern about the welfare of civilians on both sides.

"The Security Council members called for de-escalation of the situation, restoration of calm and reinstitution of the November 2012 ceasefire," the 15-member body said.

A mosque in the central Gaza Strip had been bombed to rubble, residents said. The Israeli military said the mosque had housed a weapons cache. Eight other mosques have been damaged by bombing and 537 Gaza houses have either been destroyed or damaged, Gaza-based Al-Mezan Association for Human Rights said.

By Saturday, no Israeli had been killed by the rocket salvoes out of Gaza, due in part to Iron Dome, a partly U.S.-funded interceptor system.

Racing for shelter has become a daily routine for hundreds of thousands of Israelis, and some 20,000 reservists have been mobilised for a possible thrust into Gaza.

"In the past week, we carried out a very complex technological exercise to deliver the eighth (Iron Dome) system," a Defence Ministry official said on Israel Radio.

Israel said it was determined to end cross-border rocket attacks, which intensified last month after its forces arrested hundreds of activists from the Islamist Hamas movement in the West Bank after the abduction there of three Jewish teenagers who were later found killed. A Palestinian youth was then killed in Jerusalem in a suspected revenge attack by Israelis.

"PREPARING FOR ALL POSSIBILITIES"

Asked if Israel might move from the mostly aerial attacks of the past four days into a ground war in Gaza, Prime Minister Benjamin Netanyahu said: "We are weighing all possibilities and preparing for all possibilities."

Casualties on both sides would probably rise sharply if Israeli forces stormed the largely urbanised enclave. A ground invasion of Gaza would be the first since a three-week war with Hamas in 2008-09 in which 1,400 Palestinians and 13 Israelis were killed.

Egypt's state news agency said that Egyptian President Abdel Fattah al-Sisi had met with Tony Blair, envoy for the so-called Quartet of United Nations, EU, Russia and United States, in efforts to secure a truce.

An Israeli government official said Blair had met with Netanyahu on Friday. "There are no serious contacts toward a truce. There are many proposals, but as long as Hamas keeps firing, Israel will keep fighting and will not discuss a truce."

Cairo played a crucial role in mediating a truce that ended an eight-day war between Hamas and Israel in 2012, when Egypt was governed by Hamas's Muslim Brotherhood allies.

Egypt's current military-backed government is locked in a feud with Hamas over the group's alleged support for jihadi militants in Egypt's Sinai desert - which Hamas denies. This could complicate Cairo's efforts at mediation.

Hamas spokesman Sami Abu Zuhri said: "We will not beg for calm and we continue to defend our people. Once we are offered a genuine, coherent and serious proposal, we will look into it."

Gaza medical officials said at least 82 civilians, including 25 children, were among the 125 dead so far from air strikes on the territory into which nearly 2 million people are packed.

Three militants and 15 other people, including two disabled women at a rehabilitation centre and a 65-year-old man, were killed by air strikes early on Saturday, doctors there said. An Israeli military spokeswoman had no immediate comment.

One of the dead in an air strike that killed six people in a Gaza street was identified as the nephew of Ismail Haniyeh, the Hamas political leader in the territory.

"MAP OF PAIN"

Israel says Hamas puts innocent Gazans in harm's way by placing weaponry and gunmen in residential areas. A senior Israeli military officer said aircraft had aborted "hundreds" of strikes to avoid collateral damage and that targets bombed were meant to impact Hamas fire capacity.

"We are dealing with a variety of families of targets. If there is a kind of a map, or a map of pain that the enemy sees, we create a lot of pain so that he will have to think first to stop the conflict," the officer said in a briefing to reporters.

Israel says it has hit more than 1,000 targets in Gaza.

(Additional reporting by Dan Williams in Tel Aviv and Ali Abdelatti and Stephen Kalin in Cairo; Editing by Mark Heinrich and Lynne O'Donnell)

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